1. Field of the Invention
This invention relates to a surface-mounted electromagnetic sound generator in which sound is generated by vibrating a vibration plate using an electromagnet. More particularly, the invention relates to an electromagnetic sound generator in which a yoke used in the electromagnet is insert-molded when the case of the sound generator is molded.
2. Description of the Related Art
A yoke employed in a surface-mounted electromagnetic sound generator of this kind according to the prior art usually is fixed within a case by means of a bonding agent. In order to reduce the thickness of the case, however, a yoke 4 is integrated with a case 2 made of plastic, as shown in FIGS. 3 and 4. Specifically, when a lower case 2b of the box-shaped case 2 is molded, a magnetic-circuit board 4a of the yoke 4 is imbedded by being insert-molded into side walls 2c and a bottom 2d of the case, as shown in FIG. 3. As a result, the magnetic-circuit board 4a is secured by being integrally molded with the lower case 2b.
When the case 4 is integrated with the lower case 2b in the manner described above, the bottom 2d of the lower case 2b is capable of being reinforced by the magnetic-circuit board 4a of the yoke 4. This makes it possible to reduce the thickness T of the bottom 2d. As a result, it is possible to reduce the thickness of the case housing the surface-mounted electromagnetic sound generator having this structure.
In the example of the prior art described above, it is possible to reduce the thickness of the bottom 2d of lower case 2b. However, if the thickness T is reduced to 0.3 mm, the bottom 2d may not be filled with molding material adequately when the lower case 2b is injection molded. Molding can be difficult to carry out. Consequently, even it is attempted to achieve a thinner device by thinning the bottom 2d of the lower case 2b, a major reduction in thickness is not possible.
Further, problems arise in a case where it is attempted to deal with this situation by insert-molding the yoke. Specifically, the thickness of both the side wall 2c and bottom 2d of the lower case 2b is small and, in addition, the magnetic-circuit board 4a of the yoke 4 is insert-molded with its outer peripheral portion being only slightly inserted into the side wall 2c. The result is that the yoke 4 is surrounded only by a molded portion of small thickness. This means that it is difficult to acquire sufficient strength to fix the yoke 4 and to provide enough strength to resist impact. A high strength is required especially since a coil 6, a ring-shaped magnet 8 and a vibration-plate supporting ring 12, which supports a vibration plate 10, are attached to the yoke 4.